Method of manufacturing optical fiber base material

ABSTRACT

A tube-shaped, GeO 2 -doped silica glass pipe having a high refractive index is prepared (step S 2 ), a cylindrical silica glass rod doped with F element is inserted inside the GeO 2 -doped silica glass pipe (step S 3 ), and the GeO 2 -doped silica glass pipe and silica glass rod in thus inserted state are heated so as to be unified (step S 4 ). Further, thus formed assembly is inserted inside the a pure silica glass pipe (step S 5 ), and they are heated so as to be unified (step S 6 ), whereby an optical fiber preform is manufactured.

TECHNICAL FIELD

[0001] The present invention relates to a method of making an opticalfiber preform from which an optical fiber can be obtained by drawing.

BACKGROUND ART

[0002] Dispersion-shifted optical fibers have a zero-dispersionwavelength, where the wavelength dispersion value becomes zero, in thevicinity of 1.55 μm; one kind of which has a ring core region with ahigh refractive index disposed around a center core region, and acladding region disposed around the ring core region. Adispersion-shifted optical fiber having such a refractive index profileof a ring type structure is manufactured by drawing an optical fiberpreform having a similar refractive index profile.

[0003] Such a refractive index profile of an optical fiber preform canbe realized by the center core portion made of silica glass which is tobecome the center core region of the optical fiber and the ring coreportion made of silica glass including GeO₂ (germanium dioxide) which isto become the ring core region of the optical fiber. In general, theoptical fiber preform is manufactured by VAD (vapour-phase axialdeposition) method, OVD (outside vapour deposition) method, or the like.

DISCLOSURE OF THE INVENTION

[0004] When the optical fiber preform having the above-mentionedrefractive index profile is manufactured by VAD method or OVD method,while gases for doping F element such as CF₄ gas, SiF₄ gas, and the likeare introduced so as to add F element to the center core portion, Felement is also doped in the ring core portion. If F element is doped inthe ring core portion which should have a high refractive index, theoverall refractive index will decrease, so that a desirable refractiveindex profile cannot be attained, whereby desirable fibercharacteristics cannot be obtained. If the GeO₂ concentration in thering core portion is enhanced as measures against this problem, thetransmission loss in the optical fiber will increase, and nonlinearoptical phenomena will be more likely to occur.

[0005] In order to eliminate the foregoing problems, it is an object ofthe present invention to provide a method of making an optical fiberpreform which can favorably make an optical fiber preform having arefractive index profile of a ring type structure whose center coreportion is doped with F element.

[0006] The optical fiber making method in accordance with the presentinvention comprises: (1) an insertion step of inserting a silica glassrod having a cylindrical form doped with F element inside a tubularsilica glass pipe including a region with a refractive index greaterthan that of pure silica glass; and (2) a heat-collapsing step ofheating and collapsing the silica glass pipe and silica glass rod whichare brought into an inserted state by the insertion step.

[0007] According to this optical fiber making method, while a desirableamount of F element is doped in the silica glass rod which is to becomethe center core region having a low refractive index in the opticalfiber, no F element is doped in the silica glass pipe which is to becomethe ring core region having a high refractive index in the opticalfiber, whereby an optical fiber preform having a refractive indexprofile of a desirable ring type structure can easily be manufactured.

[0008] Preferable as the silica glass pipe in the optical fiber makingmethod in accordance with the present invention is one prepared by: (a)a soot body synthesizing step of synthesizing a soot body of GeO₂ andSiO₂ on an outer peripheral surface of a starting pipe; (b) atransparent glass forming step of dehydrating and consolidating the sootbody on the outer peripheral surface of the starting pipe so as to forma transparent glass body; and (c) a removing step of removing theportion of the starting pipe in the transparent glass body so as toprepare the silica glass pipe. Alternatively, preferable is the silicaglass pipe prepared by: (a) a soot body synthesizing step ofsynthesizing a rod-shaped soot body of GeO₂ and SiO₂; (b) a transparentglass forming step of dehydrating and consolidating the soot body so asto form a transparent glass body; and (c) boring step of boring thetransparent glass body along a center axis thereof so as to prepare thesilica glass pipe. Also, preferable is the silica glass pipe preparedby: (a) a soot body synthesizing step of synthesizing a soot body ofGeO₂ and SiO₂ on an outer peripheral surface of a starting rod; (b) atransparent glass forming step of dehydrating and consolidating the sootbody so as to form a transparent glass body; and (c) a removing step ofremoving the portion of the starting rod in the transparent glass bodyso as to prepare the silica glass pipe. Further,preferable is the silicaglass pipe prepared by: (a) a soot body synthesizing step ofsynthesizing a rod-shaped soot body of GeO₂ and SiO₂; (b) a transparentglass forming step of dehydrating and consolidating the soot body so asto form a transparent glass body; and (c) a pipe forming step of heatingthe transparent glass body to a softening temperature or higher andinserting a rod having a melting point higher than the softeningtemperature into the transparent glass body along the center axisthereof so as to prepare the silica glass pipe. According to any ofthese preparing methods, a silica glass pipe, doped with GeO₂, having arefractive index higher than that of pure silica glass is prepared.

[0009] The present invention will be more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only and are not to be consideredas limiting the present invention.

[0010] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a view for explaining the refractive index profile of anoptical fiber obtained by drawing an optical fiber preform which isfavorably manufactured by the optical fiber preform making method inaccordance with the present invention;

[0012]FIG. 2 is a flowchart for explaining a preferred embodiment of theoptical fiber preform making method in accordance with the presentinvention;

[0013]FIGS. 3A to 3F are transverse sectional views of the silica glasspipe and silica glass rod at the respective times when the individualsteps in FIG. 2 are terminated;

[0014]FIG. 4 is a flowchart for explaining a first embodiment of themethod of preparing a silica glass pipe;

[0015]FIGS. 5A to 5C are transverse sectional views of the silica glasspipe at the respective times when the individual steps in FIG. 4 areterminated;

[0016]FIG. 6 is a flowchart for explaining a second embodiment of themethod of preparing a silica glass pipe;

[0017]FIGS. 7A to 7C are transverse sectional views of the silica glasspipe at the respective times when the individual steps in FIG. 6 areterminated;

[0018]FIG. 8 is a flowchart for explaining a third embodiment of themethod of preparing a silica glass pipe;

[0019]FIGS. 9A to 9D are transverse sectional views of the silica glasspipe at the respective times when the individual steps in FIG. 8 areterminated;

[0020]FIG. 10 is a flowchart for explaining a fourth embodiment of themethod of preparing a silica glass pipe;

[0021]FIGS. 11A to 11D are longitudinal sectional views of the silicaglass pipe at the respective times when the individual steps in FIG. 10are terminated; and

[0022]FIGS. 12A to 12D and 13A to 13C are views for explaining otherrefractive index profiles of the optical fiber preform favorablymanufactured by the optical fiber preform making method in accordancewith the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0023] In the following, embodiments of the present invention will beexplained in detail with reference to the accompanying drawings. Tofacilitate the comprehension of the explanation, the same referencenumerals denote the same parts, where possible, throughout the drawings,and a repeated explanation will be omitted.

[0024] To begin with, an optical fiber obtained by drawing an opticalfiber preform manufactured by the optical fiber preform making method inaccordance with the present invention will be explained. FIG. 1 is aview for explaining the refractive index profile of this optical fiber.The refractive index profile indicates the refractive index distributionin a direction of a transverse cross section orthogonal to the opticalaxis of the optical fiber.

[0025] The optical fiber shown in this figure has a center core region11 (with an outside diameter 2 a) having a refractive index n1 at acenter area including the optical axis, a ring core region 12 (with anoutside diameter 2 b) having a refractive index n2 therearound, and acladding region 13 (with an outside diameter 2 c) having a refractiveindex n3 therearound. The refractive index n2 of the ring core region 12is greater than each of the refractive index n1 of the center coreregion 11 and the refractive index n3 of the cladding region 13. Theoutside diameter 2 a of the center core region 11 is several μm, theoutside diameter 2 b of the ring core region 12 is on the order ofseveral μm to 10 μm, and the outside diameter 2 c of the cladding region13 is usually 125 μm. On the other hand, the relative refractive indexdifference of the center core region 11, Δn⁻=(n1−n3)/n3, with referenceto the refractive index n3 of the cladding region 13 is within the rangeof −1% to 0%; whereas the relative refractive index difference of thering core region 12, Δn⁺=(n2−n3)/n3, similarly with reference to therefractive index n3 of the cladding region 13 is on the order of 0.2% to1.5%. An optical fiber preform from which an optical fiber having such arefractive index profile is prepared by drawing also has a refractiveindex profile similar thereto.

[0026] A preferred embodiment of the optical fiber preform making methodin accordance with the present invention will now be explained withreference to FIGS. 2 and 3A to 3F. FIG. 2 is a flowchart for explainingthe optical fiber preform making method in accordance with thisembodiment. FIGS. 3A to 3F are sectional views of a silica glass pipe 20and a silica glass rod 10, which are a starting material or intermediateproduct of the optical fiber preform, at the respective times when theindividual steps of the optical fiber preform making method inaccordance with this embodiment are terminated.

[0027] Initially, at step S1, a silica glass rod 10 such as the oneshown in FIG. 3A is prepared. This silica glass rod 10 is to become thecenter core region 11 with a low refractive index in the optical fiberhaving the refractive index profile shown in FIG. 1, and is, forexample, shaped like a cylinder having a diameter of about 40 mm and alength of about 300 mm, mainly composed of SiO₂ glass, and doped with Felement. Its F element concentration is on the order of 0.1 wt % to 2 wt%. This silica glass rod 10 is manufactured, for example, by preparing asoot body of SiO₂ by known VAD method, and then dehydrating andconsolidating this soot body in an atmosphere of a mixed gas composed ofSiF₄, Cl₂, He, and the like so as to form transparent glass. Doping withF element can cause the silica glass rod 10 to have a refractive indexlower than that of pure SiO₂ glass. The following table summarizesdifferences caused by F element concentrations in the relativerefractive index difference Δn_(r)=(n_(r)−n_(st))/n_(st) of the silicaglass rod 10 with reference to pure SiO₂ glass, where the refractiveindex of pure SiO₂ glass is n_(st) and the refractive index of thesilica glass rod 10 is n_(r). Relative refractive index F elementdifference Δn_(r) 0.6 wt % −0.2% 1.1 wt % −0.4% 1.4 wt % −0.5% 2.0 wt %−0.7%

[0028] Also, at step S2, a silica glass pipe 20 such as the one shown inFIG. 3B is prepared. This silica glass pipe 20 is to become the ringcore region 12 with a high refractive index in the optical fiber havingthe refractive index profile shown in FIG. 1, and is, for example,shaped like a tube having an inside diameter of about 10 mm, an outsidediameter of about 24 mm, and a length of about 300 mm, mainly composedof SiO₂ glass, and doped with GeO₂. Its GeO₂ concentration is on theorder of 0.3 mol % to 30 mol %. Doping with GeO₂ can cause the silicaglass pipe 20 to have a refractive index higher than that of pure SiO₂glass. The following table summarizes differences caused by GeO₂concentrations in the relative refractive index differenceΔn_(p)=(n_(p)−n_(st))/n_(st) of the silica glass pipe 20 with referenceto pure SiO₂ glass, where the refractive index of the silica glass pipe20 is n_(p). Relative refractive index GeO₂ Concentration differenceΔn_(p) 11 mol % 1.1% 12 mol % 1.2% 15 mol % 1.5%

[0029] The method of preparing the silica glass pipe 20 will beexplained later. Here, steps S1 and S2 may be carried out in the reverseorder or in parallel separately from each other.

[0030] At step S3, as shown in FIG. 3C, the silica glass rod 10 isinserted into the silica glass pipe 20. Here, it does not matter if agap is formed between the silica glass pipe 20 and the silica glass rod10. Nevertheless, for obtaining an optical fiber in which theellipticity of the center core region 11 is low, the gap is preferablyas small as possible. Before the insertion, one or both of the silicaglass pipe 20 and the silica glass rod 10 may be heated to elongateuntil an appropriate diameter is attained, or one or both of the innerperipheral surface of the silica glass pipe 20 and the outer peripheralsurface of the silica glass rod 10 may be surface-treated with an HFsolution. Here, in the case where the silica glass rod 10 is elongatedby heating with an oxygen/hydrogen flame, the surface treatment with theHF solution is essential to removing the moisture adhering to thesurface of the silica glass rod 10.

[0031] At step S4, the silica glass pipe 20 assembly with the silicaglass rod 10 inserted therein at step S3 is heated, so as to be unifiedas shown in FIG. 3D. This heat-collapsing step is carried out in anatmosphere of Cl₂ gas or of a mixed gas of Cl₂ and O₂.

[0032] At step S5, the silica glass pipe 20 and the silica glass rod 10unified by heating at step S4 are inserted into a pure silica glass pipe30 as shown in FIG. 3E. The pure silica glass pipe 30 is to become thecladding region 13 having a low refractive index in the optical fiber.Before the insertion, the unified silica glass pipe 20 and silica glassrod 10 may be heated to elongate until an appropriate diameter isattained, or the outer peripheral surface thereof may be surface-treatedwith an HF solution. Here, in the case where the silica glass pipe 20and silica glass rod 10 unified by heating are elongated with anoxygen/hydrogen flame, the surface treatment with the HF solution isessential to removing the moisture adhering to the surface of theheat-unified silica glass pipe 20 and silica glass rod 10.

[0033] Then, at step S6, the assembly prepared at step S5 by insertingthe heat-unified silica glass rod 10 and silica glass pipe 20 into thepure silica glass pipe 30 is heated, so as to be unified as shown inFIG. 3F. Hence, the making of an optical fiber preform is completed.Thus manufactured optical fiber preform has a refractive index profilesimilar to that of the optical fiber shown in FIG. 1.

[0034] According to this optical fiber making method, while a desirableamount of F element is added to the silica glass rod 10 which is tobecome the center core region having a low refractive index in anoptical fiber, no F element is added to the silica glass pipe 20 whichis to become the ring core region having a high refractive index in theoptical fiber, whereby an optical fiber preform having a refractiveindex profile of a desirable ring type structure can easily be made.

[0035] Next, four embodiments of the method of preparing the silicaglass pipe 20 will be explained in the following.

[0036] A first embodiment of the method of preparing the silica glasspipe 20 is as follows. FIG. 4 is a flowchart for explaining the firstembodiment, whereas FIGS. 5A to 5C are transverse sectional views of thesilica glass pipe 20 at the respective times when the individual stepsin the first embodiment are terminated.

[0037] To begin with, at step S11, as shown in FIG. 5A, a silica glasspipe having an inside diameter of 10 mm and an outside diameter of 20mm, for example, is employed as a starting pipe 21, and a soot body 20Amade of GeO₂ and SiO₂ is formed on the outer peripheral surface of thestarting pipe 21 by VAD method. Subsequently, at step S12, it isdehydrated and consolidated so as to form a transparent glass body asshown in FIG. 5B, which is then heated to elongate. Further, at stepS13, the portion of the starting pipe 21 in the transparent glass bodyis removed by being dissolved with an HF solution having a concentrationof 5% to 50% or by vapour-phase etching with SF₆ gas or the like, so asto prepare the silica glass pipe 20 having an inside diameter of 12 mm,an outside diameter of 24 mm, and a length of 300 mm as shown in FIG.5C.

[0038] Here, with a silica glass pipe employed as a starting pipe, asoot body made of GeO₂ and SiO₂ may be synthesized on the innerperipheral surface of the starting pipe, so as to form transparentglass, and the outer periphery may be cut by about several mm from thesurface thereof so as to remove the part mixed with OH group, thusleaving the silica glass pipe 20. In this case, it is not necessary toremove the whole starting pipe, and the remaining starting pipe portionbecomes the cladding region 13.

[0039] A second embodiment of the method of preparing the silica glasspipe 20 is as follows. FIG. 6 is a flowchart for explaining the secondembodiment, whereas FIGS. 7A to 7C are transverse sectional views of thesilica glass pipe 20 at the respective times when the individual stepsin the second embodiment are terminated.

[0040] To begin with, at step S21, a rod-shaped soot body 20B having,for example, an outside diameter of 40 mm and a length of 300 mm, madeof GeO₂ and SiO₂, such as the one shown in FIG. 7A, is formed by VADmethod. Subsequently, at step S22, it is dehydrated and consolidated soas to form a transparent glass body 20C such as the one shown in FIG.7B. Further, at step S23, an ultrasonic borer or the like is used tobore the transparent glass body 20C along its center axis, so as to forma hole having an inside diameter of 10 mm, thereby yielding a pipe,which is then heated to elongate, thus preparing the silica glass pipe20 having an inside diameter of 6 mm and an outside diameter of 24 mm asshown in FIG. 7C.

[0041] A third embodiment of the method of preparing the silica glasspipe 20 is as follows. FIG. 8 is a flowchart for explaining the thirdembodiment, whereas FIGS. 9A to 9D are transverse sectional views of thesilica glass pipe 20 at the respective times when the individual stepsin the third embodiment are terminated.

[0042] To begin with, at step S31, a starting rod 22 is prepared, and asoot body 20D made of GeO₂ and SiO₂ is formed on the outer peripheralsurface of the starting rod 22 by VAD method as shown in FIG. 9A.Subsequently, at step S32, it is dehydrated and consolidated so as toform a transparent glass body as shown in FIG. 9B. At step S33, anultrasonic borer or the like is used so as to bore the portion of thestarting rod 22 in the transparent glass body along its center, so as toform a hole having an inside diameter slightly smaller than the outsidediameter of the starting rod 22, thereby forming a pipe as shown in FIG.9C. At step S34, the remaining part of the starting pipe 22 in thetransparent glass body is removed as shown in FIG. 9D by being dissolvedwith an HF solution having a concentration of 5% to 50% or byvapour-phase etching with SF₆ gas or the like. Thus, the silica glasspipe 20 is prepared.

[0043] A fourth embodiment of the method of preparing the silica glasspipe 20 is as follows. FIG. 10 is a flowchart for explaining the fourthembodiment, whereas FIGS. 11A to 11D are longitudinal sectional views ofthe silica glass pipe 20 at the respective times when the individualsteps in the fourth embodiment are terminated.

[0044] To begin with, a soot body is formed as shown in FIG. 11A at stepS41 by VAD method in the same manner as the synthesizing of a soot bodyfor single-mode fiber, and is further heated at step S42 so as to becometransparent glass, thereby yielding a silica glass rod having a region20F (which becomes a ring core region 12) doped with GeO₂ in thevicinity of the axial center of a pipe-shaped silica glass region 20E asshown in FIG. 11B. At step S43, this glass rod is heated to at least1500° C., which is a softening temperature slightly lower than themelting point, in an inactive gas atmosphere, and then a rod 30 made ofa heat-resistant material such as tungsten, alumina, or carbon, forexample, is inserted therein along its center axis as shown in FIG. 11,so as to plastically deform the glass rod, thereby forming an openingalong the center axis (piercing), thus yielding a silica glass pipe suchas the one shown in FIG. 11D. At step S44, the surface of this silicaglass pipe is smoothed by removing at least 10 μm of the innerperipheral surface upon being dissolved with an HF solution having aconcentration of 5% to 50%, or upon heating the silica glass pipe to1000° C. or higher and then treating it by vapour-phase etching with SF₆gas or the like. Hence, the silica glass pipe 20 having an inner surfaceroughness of 10 μm or less is obtained after the treatment. Here,instead of carrying out steps S41 and S42, a glass layer of SiO₂-GeO₂may be deposited on the inner peripheral surface of a silica glass pipeby MCVD method, and the rod 30 may be inserted into this pipe so as toenlarge the opening.

[0045] The inventors produced various glass pipes according to thefourth embodiment of the glass pipe preparing method by using variousglass pipes of FIG. 11B in which the outside diameter ranges from 20 mmto 150 mm and the ratio of outside diameter of portion 20E/outsidediameter of portion 20F ranges from 1.1 to 20. As a result, it has beenconfirmed that, in thus obtained hollow glass pipes, the eccentricity((maximum thickness−minimum thickness) /outside diameter) is less than1%, the outside diameter fluctuation of the region 20F in thelongitudinal direction is 5% or less, the ellipticity of each of theinside diameter and outside diameter is 1% or less, and the hollow glasspipes can be produced with a favorable precision.

[0046] Next, the results of evaluation of various characteristics of anoptical fiber obtained by drawing an optical fiber preform manufacturedaccording to the optical fiber preform making method in accordance withthe present invention will be explained.

[0047] Here, to be used as the silica glass rod 10, one having an Felement concentration of 1.1 wt % and a relative refractive indexdifference Δn⁻ of −0.4% was elongated to yield an outside diameter of8.5 mm by heating with an electric furnace and then processed to have anoutside diameter of 8 mm with an HF solution having a concentration of10%. The silica glass pipe 20 was one prepared according to theabove-mentioned first embodiment, having a GeO₂ concentration of 12 mol%, a relative refractive index difference Δn⁺ of 1.2%, an insidediameter of 12 mm, and an outside diameter of 24 mm. In a state wherethe silica glass rod 10 was inserted into the silica glass pipe 20, theywere heated in an atmosphere of Cl₂ gas or of a mixed gas of Cl₂ gas andO₂ gas, so as to be unified. The rod made by thus unified silica glassrod 10 and silica glass pipe 20 was heated so as to elongate to have anoutside diameter of 10 mm, and then was surface-treated with an HFsolution so as to yield an outside diameter of 6.2 mm. It was insertedinto a pure silica glass pipe 30, and heated so as to be unified,whereby an optical fiber preform was made. Then, this optical fiberpreform was drawn, so as to yield an optical fiber having an outsidediameter of 125 μm.

[0048] The outside diameter 2 a of the core region of thus obtainedoptical fiber was 4.9 μm, whereas the outside diameter 2 b of its ringcore region was 7.5 μm. Various characteristics of this optical fiberwere as follows. The transmission loss at a wavelength of 1550 nm was0.21 dB/km, mode field diameter was 7.4 μm, zero-dispersion wavelengthwas 1580 nm, wavelength dispersion slope was 0.088 ps/nm²/km, effectivecross-sectional area was 86 μm², bending loss at a diameter of 20 mm was0.02 dB/m, and polarization mode dispersion was 0.08 ps/km^(½). Hence, adispersion-shifted optical fiber having a zero-dispersion wavelength inthe vicinity of 1.55 μm, small transmission loss and bending loss, and alarge effective cross-sectional area was obtained. Namely, it has beenconfirmed that an optical fiber having excellent characteristics, whichis not prone to generate nonlinear optical phenomena, can favorably beprepared from an optical fiber preform manufactured by use of the methodof making an optical fiber preform in accordance with the presentinvention.

[0049] Without being restricted to the above-mentioned embodiments, thepresent invention can be modified in various manners. The refractiveindex profile of the optical fiber preform manufactured by the method ofmaking an optical fiber preform in accordance with the presentinvention, i.e., the refractive index profile of the optical fibermanufactured by use of this optical fiber preform is not limited to thatshown in FIG. 1. FIGS. 12A to 12D show other refractive index profilesof the optical fiber preform which can favorably be prepared by themethod of making an optical fiber preform in accordance with the presentinvention, respectively. For example, as shown in FIGS. 12A and 12B, therefractive index profile of the ring core region 12 may varydiametrically instead of being uniform. Such a ring core region 12 canbe formed by synthesizing a soot body while changing the ratio betweenGeO₂ and SiO₂ when manufacturing the silica glass pipe 20 according tothe above-mentioned first to fourth embodiment. Also, as shown in FIG.12C, the refractive index profile of the inner cladding region 13 mayvary diametrically instead of being uniform. An optical fiber preformhaving such a refractive index distribution can be made by synthesizinga soot body made of GeO₂ and SiO₂ on the inner surface of a silica glasstube having such a refractive index distribution. Further, as shown inFIG. 12D, the refractive index of the center core region 11 maydiametrically change like a curve. For making an optical fiber preformhaving such a center core region 11, it will be sufficient if a glassrod having such a refractive index distribution is used as the silicaglass rod 10.

[0050] Further, not only one ring core region 12 as shown in FIGS. 1 and13A, but also a plurality of layers of ring core regions may be disposedas shown in FIGS. 13B and 13C. Optical fiber preforms having any ofthese refractive index profiles can favorably be manufactured by theoptical fiber preform making method in accordance with the presentinvention. For making an optical fiber preform having such a refractiveindex profile, a silica glass pipe with an inside diameter of 2 a and anoutside diameter of 2 b having its corresponding refractive indexprofile is used. For preparing the silica glass pipe 20 having such arefractive index profile, it will be sufficient if a soot body isdeposited and synthesized while the mixing ratio of GeO₂, SiO₂, and Felement is changed such that the concentrations of GeO₂ is increasedwhen a region which is required to have a higher refractive index isformed and F element is increased when a region which is required tohave a lower refractive index is formed.

Industrial Applicability

[0051] In accordance with the present invention, an optical fiberpreform having a refractive index profile of a desirable ring typestructure can easily be manufactured. As a consequence, an optical fibermade therefrom by drawing can also obtain a desirable refractive indexprofile, without its ring portion being doped with GeO₂ in excess, thetransmission loss of the optical fiber is small, and nonlinear opticalphenomena are hard to occur. In particular, the present invention isfavorably applicable to the making of an optical fiber preform forpreparing a dispersion-shifted optical fiber.

1. A method of making an optical fiber preform comprising: an insertionstep of inserting a silica glass rod having a cylindrical form dopedwith F element inside a tubular silica glass pipe including a regionwith a refractive index greater than that of pure silica glass; and aheat-collapsing step of heating and collapsing said silica glass pipeand said silica glass rod which are in an inserted state brought into bysaid insertion step.
 2. A method of making an optical fiber preformaccording to claim 1, further comprising: a soot body synthesizing stepof synthesizing a soot body of GeO₂ and SiO₂ on an outer peripheralsurface of a starting pipe; a transparent glass forming step ofdehydrating and consolidating said soot body on the outer peripheralsurface of said starting pipe so as to form a transparent glass body;and a removing step of removing the portion of said starting pipe insaid transparent glass body so as to prepare said silica glass pipe. 3.A method of making an optical fiber preform according to claim 1,further comprising: a soot body synthesizing step of synthesizing arod-shaped soot body of GeO₂ and SiO₂; a transparent glass forming stepof dehydrating and consolidating said soot body so as to form atransparent glass body; and boring step of boring said transparent glassbody along the center axis thereof so as to prepare said silica glasspipe.
 4. A method of making an optical fiber preform according to claim1, further comprising: a soot body synthesizing step of synthesizing asoot body of GeO₂ and SiO₂ on an outer peripheral surface of a startingrod; a transparent glass forming step of dehydrating and consolidatingsaid soot body so as to form a transparent glass body; and a removingstep of removing the portion of said starting rod in said transparentglass body so as to prepare said silica glass pipe.
 5. A method ofmaking an optical fiber preform according to claim 1, furthercomprising: a soot body synthesizing step of synthesizing a rod-shapedsoot body of GeO₂ and SiO₂; a transparent glass forming step ofdehydrating and consolidating said soot body so as to form a transparentglass body; and a pipe forming step of heating said transparent glassbody to a softening temperature or higher and inserting a rod having amelting point higher than said softening temperature into saidtransparent glass body along the center axis thereof so as to preparesaid silica glass pipe.